Monoclonal antibody (MAb) to pneumococcal type 9V polysaccharide (PS) was produced using PS conjugated with inactivated pneumolysin immunogen. Type 9V PS MAb contained subclass IgG1 antibody. The 9V PS MAb recognized and reacted with an immunologic epitope of group 9 PS in patterns which are different from that observed in rabbit 9V antiserum (AS). The change of antigen-antibody reaction rate as measured by rate nephelometry in the group 9 PS-9V MAb reaction increased rapidly at low concentrations and reached a plateau at 10 ug PS/ml. In contrast, the change of reaction rate in group 9 PS-rabbit 9V AS reaction increased proportionally and continued to increase at the concentration of 20 ug PS/ml. In immunodiffusion, 9V MAb reacted with 9N PS whereas there was no precipitin reaction between rabbit 9V AS and 9N PS. Furthermore,the 9V PS MAb is both opsonic and protective against challenge with type 9 pneumococci in young mice. These results indicate that the passive immunity combined with active immunization could induce effective protective immunity for prevention of pneumococcal infection in young children.